Excited Orbitals Research Articles

ChemInform Abstract: Electron Energy Loss Spectroscopy of the Outer Valence Shells of Acetonitrile and Methyl Isocyanide.

Valence-shell electronic excitation of acetonitrile and methyl isocyanide has been measured by electron energy loss spectroscopy at 20-, 25-, and 70-eV impact energies. The acetonitrile spectra are discussed in terms of valence-state excitation in the 5-9.5-eV region and Rydberg series converging to the first and to the second ionization limits for the 9.5-12-eV energy loss range. For some of these transitions new assignments are proposed. The relative differential cross sections for the most intense Rydberg transitions have been measured as a function of the scattering angle (6-90{degree}). They lead to a discussion of the symmetry of the excited orbitals. The spectrum of methyl isocyanide has been recorded in the 6-13-eV region at 25 eV and 2{degree}.

Electron energy loss spectroscopy of the outer valence shells of acetonitrile and methyl isocyanide

Valence-shell electronic excitation of acetonitrile and methyl isocyanide has been measured by electron energy loss spectroscopy at 20-, 25-, and 70-eV impact energies. The acetonitrile spectra are discussed in terms of valence-state excitation in the 5-9.5-eV region and Rydberg series converging to the first and to the second ionization limits for the 9.5-12-eV energy loss range. For some of these transitions new assignments are proposed. The relative differential cross sections for the most intense Rydberg transitions have been measured as a function of the scattering angle (6-90{degree}). They lead to a discussion of the symmetry of the excited orbitals. The spectrum of methyl isocyanide has been recorded in the 6-13-eV region at 25 eV and 2{degree}.

Vacuum ultraviolet photochemistry of CHFCl2 and CHFBr2: Absorption spectra and CHF(Ã 1A″) radical formation

Photoabsorption and fluorescence excitation spectra of CHFCl2 and CHFBr2 were measured in the wavelength region 106–200 nm using synchrotron radiation as a light source. The Rydberg excitations of the outer shell orbitals are discussed. The He(i) photoelectron spectrum of CHFBr2 was first measured. Diffuse emission band of CHF(à 1A″→X̃ 1A′) was observed in the 350–700 nm region, in which the 2n0 (n=0–4) vibrational progression and 201 transition were identified. The CClF(à 1A″) radical was produced together with CHF(à 1A″) in the photodecomposition of CHFCl2. The radiative lifetimes of the à 1A″ states of CHF and CClF were determined to be 2.57±0.16 μs and 644±15 ns, respectively. In the photolysis of CHFCl2 fluorescence starts at 155 nm and shows a maximum quantum yield Φ=0.22 at 107.5 nm, while the CHFBr2 excitation gives a maximum of Φ=0.016 at 117.5 nm.

THEORETICAL STUDY ON THE MOLECULAR NEAR-THRESHOLD STRUCTURE

Excited molecular structure in gas phase usually consists of two types of energy states: Rydberg states with diffused molecular orbitals and valencetype excited states with rather localized molecular orbitals. Excitation from molecular inner-shell orbitals into the above-men-rioned excited orbitals leads to near-threshold structures. Based on the multiple-scattering self-consistent field theory, we have studied the molecular near-threshold structure. Our theoretical results are in good agreement with experimental measurements. We have also elucidated:(1) The dynaimcal interaction between the excited electron with the residual molecular ion.(2) Physical orgin of the shaperesonance, namely, as an anti-bonding state embedded in the continuum.

Successive embeddings of excited atomic dipoles in plasmas

An iterative and nonlinear procedure based on Renormalization group ideas is investigated for static screened Coulomb potentials through successive inclusion of excited 2-body orbitals. The key quantities are dielectric constants for every excited state, built into a hierarchy of nonlinear differential equations. Different numerical approaches to the Schrödinger problem are contrasted. They are implemented through the analytic solutions of the given HULTHEN potential. As a result, one observes a systematic shallowing of the pair interaction altogether with a blue shift of the bound-bound transitions.

A new high-resolution study of the 3d-subshell excitation spectrum of chromium

The authors report new high-resolution 3d-photoabsorption spectra of Cr I. The object of the study was to provide further insight into the remarkable simplicity of the previously studied 3d-excitation spectra of Cr I and Mn I by making observations at much greater resolution. The authors have resolved all the K components of the lines predicted by JK coupling, but not the full multiplicity. In addition, all autoionisation widths and interchannel interactions are found to be very small. These properties of the spectra can be accounted for by the hydrogenicity of the excited np orbitals, which is shown to underpin the spectator-electrons approximation. The authors are also able to justify the hydrogenicity of the np wavefunctions by comparison with species adjacent to chromium in the periodic table.

Electronic structure and bond length dependence of the effective valence shell Hamiltonian of S2 as studied by quasidegenerate many-body perturbation theory

The effective valence shell Hamiltonian (Hv) of S2 is calculated as a function of internuclear distance using quasidegenerate many-body perturbation theory with the full valence space spanned by eight valence orbitals. Calculated potential curves and excitation energies for several valence states are in good agreement with experiment and are compared with configuration interaction calculations using the same primitive basis. In order to test assumptions of semiempirical theories, we also perform a more approximate calculation of Hv in which the valence space is constructed as the union of the atomic valence spaces with the atomic orbitals taken from atomic SCF calculations. A new and important feature of this approximate, correlated Hv is the use of optimized valence and excited orbitals as determined from a constrained SCF procedure. The matrix elements of this approximate, correlated Hv are transformed to the original nonorthogonal atomic valence basis, and their bond length dependences are fit with simple analytical functions. Some calculated Hv matrix elements agree with the forms commonly postulated for semiempirical integrals, while others display quite different behavior. An example of the latter are the one-center, two-electron integrals which depend significantly on bond length in marked contrast to semiempirical theories which assume them to be bond length independent.

Extreme sensitivity of quadrupole moments of Rn isomers for small deformations

Abstract The quadrupole moments of proton isomers in 208,210,211 Rn show a drastic increase over the N = 126 closed neutron shell isotope 212 Rn. The increase — although consistently understood in the deformed independent-particle model — only indicates a minor deformation of the core when pair scattering into excited high-spin orbitals is taken into account properly.

First- and second-order contributions to the effective Hamiltonian parameters for Gd 3+( 8S) and Mn 2+( 6S)

The effective operator formalism is used to calculate the values of effective Hamiltonian parameters which arise from higher order perturbation corrections due to the relativistic crystal field and spin-orbit interactions. The method is illustrated on two examples: the Gd 3+ ion in the C 3h-symmetry and the Mn 2+ ion in the C 3v-symmetry. Transitions to all excited orbitals for l = s, p, d, f and n = l+1,…, 10 are included in calculations. The role of second-order perturbation corrections is significant for both excitations within the ground configuration and outside it.

Localization vs. delocalization of orbitals in the quasi-one-dimensional conductor Na 0.33V 2O 5.

The beta phase of sodium vanadium oxide bronzes Na xV 2O 5 extends from x=0.22 to fx=0.4. Well inside the stability range, at x=0.33, the electrical conductivity undergoes abrupt change and the material becomes a quasi-one-dimensional conductor. We present a study of the electronic structure and excitation properties of Na 0.33V 2O 5 by x-ray photoelectron spectroscopy (XPS) and appearance potential spectroscopy (APS). While the difference between the Fermi level of the sample material (as determined by XPS) and the edge of the vanaium L 3 APS spectrum indicates strong final state many-body effects due to the localized character of the excited orbitals, the presence of a negative undershoot in the APS spectrum suggests a rather efficient screening (therefore an extended charge mobility) at the site of the excited atom. These apparently conflicting data support and illustrate the idea of domain formation in the sodium vanadium oxide bronze Na 0.33V 2O 5.

X-ray-edge problem in metals. I. Universal scaling in alkali-alkali alloys.

We report outer-core excitation spectra of alkali-alkali alloys. The results are obtained by differential reflectance methods using synchrotron radiation on quench-condensed samples. Striking changes of line shape take place with changing composition. All available threshold profiles for alkali-metal alloys and pure metals conform to a universal scaling relationship defined by parameters related to the electronic structure of the excited configuration. The sharp ``x-ray edges'' with an overshoot at threshold occur only when the excited alkali-metal-atom valence orbitals localize at or below the band bottom and other atoms of the same type neighboring the active core in the Periodic Table. A linear absorption profile rising from zero absorption at threshold is observed when the excited orbitals mix strongly with conduction-band states.

Transition probability and photoionisation cross section calculations for CH4and HF by one-centre expansion and Stieltjes technique

Transition probabilities, partial and total photoionisation cross sections for CH4 and HF are obtained from a discrete basis set calculation by using the Stieltjes imaging technique for the continuum oscillator strength densities. Bound and continuum electronic eigenfunctions in the ground-state equilibrium geometry are described in the independent channel approximation using large basis sets of integrable functions centred on the heavy atom. Besides the usual STO functions, hydrogenic Rydberg functions and diffuse oscillatory functions are used to improve the description of the bound excited orbitals and the continuum orbitals respectively. The results in the discrete and continuous energy regions compare well with the available experimental data, confirming the validity of the approach for this class of molecules.

Extended floating spherical Gaussian basis sets for Molecules. FSGO basis for use in advanced correlated calculations of electronic structures

The transformation of occupied and excited SCF orbitals expressed in Cartesian Gaussian form to a smaller, simpler set of floating spherical Gaussians is described. Illustrative applications at the correlated coupled-cluster level are presented for Lill and H 2O.

Excited state dynamics of NaF:Cu+

We have previously used one- and two-photon spectroscopy to characterize the vibronic structures observed for NaF:Cu+ in the excited states of the d 9s configuration. We were able to resolve the zero-phonon lines and a progression of the first three or four quanta of the t1u vibration (involving the off-center motion of Cu+). In the present work we model the line positions and intensities with a mixed quadratic–quartic spherical potential. The spherical potential along with a perturbative treatment of the crystal field describes the members of the 1T2g state progression well. We also semiquantitatively treat the Jahn–Teller effect, operative in the 1Eg state only. The fits indicate the existence of a radial barrier in the potential for the off-center motion of Cu+ in the 1T2g and the 1Eg states of approximate magnitude 1.9 and 73 cm−1, respectively. We also find that the Cu+ ion moves preferentially toward the fluorines rather than between them in the 1T2g state. The derived potential energy surfaces are discussed in terms of the dynamical mixing of the excited 4s and 4p orbitals of Cu+.

The effect of the excited impurity states in heavily doped semiconductors

The role of the excited impurity orbitals in doped semiconductors is discussed. It is shown that they are crucial elements for the case of n-doped Si or Ge at intermediate concentrations near the metal-non-metal transition. This is due to the fact that, for this range of concentrations, the states with energies near the Fermi level have mainly an 'excited' character. The system is described by a tight-binding Hamiltonian with a basis composed by two orbital per site. Only the local electron repulsion is considered. The correlation is treated in the coherent potential approximation. To calculate the configurational averages a diagrammatic scheme is developed. It represents an extension of the Matsubara and Toyozawa method for the case of two hybridised bands in the presence of electronic correlation.

Nonexistence of a positron-hydrogen-atom bound state

The problem of the existence of bound states for the e+-H system was investigated by A~o~soN, KLEIN~AN and SrRucI~ (1), by AR~ou~ and SCRAD~R (~) and by ARMOUR (2). These papers conclude that the fundamental state of the hydrogen atom does not support a bound state with a positron. A lower bound to the so-called critical mass of the positron is given. Although the last results are believed to be very accurate, the matter can be still considered to be settling down. For this reason, a further proof of the nonexistence of a positron-hydrogen bound state is still of interest. In this paper we confirm the nonexistence of such a bound state through the use of a simple approximate model and a direct calculation method. Both the model and the related calculation will be used in subsequent work to study the existence of positron-atom resonances. The calculation method consists in the determination of the e+-H potential with an adiabatic, close-coupling approximation. In this approximation the positron is considered to approach the atom so slowly that the atom can be considered to be fully distorted for each position of the positron. The energy levels eventually supported by the potential well are determined. The calculation scheme closely follows the one used by STON~ (4). The potential is buil t as an expansion in which the static (repulsive) term is dominant. The polarization potential is introduced as the finite sum of the terms representing the atomic distortion of several excited orbitals. The practical l imitations of the close-coupling technique are well known (4). As a matter of fact a correct expan-

Importance of relativistic interactions in photoemission of high angular momentum electrons in mercury

The authors have studied the interplay between potential barrier effects, exchange terms and relativistic interactions in photoemission processes involving high angular momentum subshells (nd5/2, nd3/2 with n=4 and 5, 4f7/2 and 4f5/2) of mercury. Ab initio calculations of various photoemission cross sections, angular distribution parameters and phaseshifts of the corresponding continuum wavefunctions are performed in jj coupling, using the Dirac-Slater approximation. The physical origin and the importance of spin-orbit effects are established in the framework of the angular momentum transfer theory which shows that, in these cases, the spin-orbit effects introduce extra values of the transferred angular momentum and manifest themselves as anisotropic interactions as the photoelectron escapes from the core. The authors show that they are greatest in the channels nl4l+1l-1, because of the existence of an intense repulsive potential barrier in the channels nl4l+1l+1. In the latter, the excited orbitals may be not penetrating enough to mix both singlets and triplets before the photoelectron energy has gained enough energy so that the influence of relativistic interactions may be delayed.

Ab initio refinement of an orbital-centred force field for biomolecules: Orbital localisation and parameterisation of the COP(O 2)OC fragment of nucleotides

Classical calculations of conformational potential surfaces, based on simple analytical functions of the interactions between atomic centres, continue to be of considerable importance. However, it has become apparent that not all interactions of importance can be included as interactions between positions which are effectively those of the nuclei. Thus, there has been recent interest in including lone pairs and special functions for interactions involving excited state orbitals. A particularly interesting test case is the COP(O 2)OC fragment of the nucleic acid backbone, which would seem to be the most flexible “hinge point” of polynucleotides when the classical type of calculation is carried out. In contrast, ab initio quantum mechanical calculations show the conformational space of this fragment to be much more restricted. The disagreement is such that it calls into doubt the validity of the bench-top modelling of nucleotide behaviour. In the following study, a variety of ab initio calculations are carried out to localise, in an objective manner, non-core orbitals. Coulombic interactions are introduced between these localised orbitals, with charge parameters optimised to reproduce the total ab initio potential surface. The results imply an interesting disgreement with other authors, concerning the importance of lone pair interactions in the nucleotide backbone, and the origins of this disagreement are analysed in some detail.

The spin-coupled valence bond theory of molecular electronic structure. I. Basic theory and application to the 2 Σ + states of BeH

The spin-coupled valence bond theory of molecular electronic structure is developed, according to which the single configuration spin-coupled theory is reformulated so as to yield both ground and excited orbitals. These orbitals are subsequently used to generate v.b. structures, the Hamiltonian matrix of which is diagonalized as in the conventional v.b. method. The fundamental feature of the excited spin-coupled orbitals is that, except those with the highest energy, they retain the characteristic distorted atomic form of the ground state orbitals, and correspondingly possess negative orbital energies. This leads to compact and rapidly convergent wavefunctions for the ground and lower-lying excited states, thus overcoming one of the basic drawbacks of the original v.b. theory. The theory is applied to the 2 ∑ + states of BeH by using 53, 71 and 80 structures of this kind. Very good convergence is found for the lowest six states, and the total energy of the ground state is below that given by a very large m.o.c.i. calculation. The present theory is thus a powerful and flexible alternative to m.o.c.i. calculations but using about an order of magnitude fewer functions.

Experimental and theoretical shakeup studies. III. The 1s shakeup in CH4, NH3, and H2O

The core 1s level x-ray photoelectron spectra of CH4, NH3, and H2O have been recorded to high statistics. Several satellite peaks are resolved in each spectrum. The shakeup peaks are assigned with the aid of Xα–SW calculations. A comparison of the ten electron hydride MO diagrams with the neon atomic energy levels reveal the Rydberg character of the empty excited state orbitals. The shakeup transitions in the respective molecules can also be interpreted from atomic models constructed from the experimental shakeup spectrum of neon. The 2p contribution to the total relaxation energy is calculated from a spectral weight analysis of the shakeup spectra.